Electrophysiological brain oscillations are often separated into distinct frequency bands, ranging from low-frequency delta (1–4 Hz) to high-frequency gamma (<40 Hz). The power and/or connectivity profiles of these frequency bands have been linked with specific neuronal and cognitive functions (Buzsáki and Draguhn, 2004; Palva et al., 2005). Whilst this has proven a powerful tool in neuroscientific research, there is emerging evidence that oscillations from different frequency bands also display specific coupling patterns—a phenomenon termed cross frequency coupling (CFC; Jensen and Colgin, 2007; Hyafil et al., 2015). One of the best studied forms of CFC is phase-amplitude coupling (PAC), in which the amplitude/power of a high frequency oscillation, often gamma (>40 Hz), is coupled to the phase of a lower frequency oscillation (Canolty et al., 2006; Canolty and Knight, 2010). PAC has been observed in multiple regions of the human brain, including the visual cortex (Voytek et al., 2010), auditory cortex (Cho et al., 2015), hippocampus (Lega et al., 2014; Heusser et al., 2016), and prefrontal cortex (Voloh et al., 2015; Voytek et al., 2015), in both electrocorticography (ECOG) and magnetoencephalography (MEG) recordings.
